1. Field of the Invention
The present invention relates to filter systems used in digital communication transmitters. More particularly, the present invention relates to an improved modulator employing direct digital synthesizers to create a designed waveform output which eliminates conventional filters in digital transmitters.
2. Description of the Prior Art
Data which is digital in nature such as NRZ, BPSK and QPSK, etc. has an infinite band width. When such data is to be transmitted, it must be limited in band width to comply with FCC requirements. The usual way of limiting the band width of such digital data at the transmitter is to filter the RF signal after the output of the modulator using band pass filters. Such RF band pass filters cannot be easily and accurately designed to control a narrow frequency band using conventional techniques. An alternative way of limiting the digital data is to filter the base band signal prior to input to the modulator. Such base band filters can only be used in conjunction with highly linear multipliers and amplifiers downstream from the digital modulator.
Accordingly, it would be highly desirable to eliminate convention signal shaping filters used in digital data transmitters.